1. Field of the Invention
This invention relates to apparatus for determining when two periodic waveforms are within a predetermined phase angle of being in phase.
2. Background Information
In certain applications, it is necessary or desirable to determine the phase angle between periodic waveforms. The detection and discrimination of the angle of phased displacement between two sinusoidal waveforms is used frequently in the design of automatic synchronization apparatus for electric power generation systems. In such synchronization apparatus, the two sinusoidal waveforms typically represent the voltages of a generator and of an electric grid to which the generator is to be connected. In particular, the concept of phase angle discrimination is used in a device generally referred to as a "synchro acceptor". The function of such a device is to describe a "window" of phase displacement in which synchronization is permitted. Such a device is typically used together with an automatic synchronizer, or a manual synchronization system, to restrict the maximum possible phase displacement at the time of closing the circuit breaker connecting the generator to the grid. In the event of a failure of the automatic apparatus, or a careless attempt to synchronize manual means, this acceptance window will limit the out-of-phase condition that can exist at the time of closure.
A typical synchro acceptor currently available is disclosed in U.S. Pat. No. 3,652,933. In this apparatus, the two incoming sinusoids are converted to square waves, and an exclusive NOR function is applied to the trains of square waves. This function produces a train of pulses of varying widths, where the width of the pulse is related to the instantaneous phase displacement. This pulse train is then filtered using conventional low-pass filtering techniques to produce a triangular waveform. The amplitude of this triangular waveform at any given time is proportional to the phase displacement of the two sinusoids. The frequency of the triangular waveform is equal to the difference between the grid and generator frequencies, and is called the "slip" frequency. The triangular waveform is input to an amplitude detector (comparator) circuit. By adjusting a trigger level of the comparator, the output of the comparator can be enabled only for amplitudes greater than the trigger level. Hence, this circuit can be made to enable synchronization only for a certain range of phase displacements. This range is usually called the "window".
It is well known that the process of low-pass filtering utilizing RC (resistor-capacitor) networks introduces phase delays and attenuates amplitude as the frequency of the input waveform increases. The process of low-pass filtering as employed by the synchro acceptor of U.S. Pat. No. 3,652,933 will be penalized by the same phase delays and variations in amplitude with frequency. Since the window is determined by the level detecting comparator, any variations in amplitude attributable to frequency will distort the resulting window.
Other filtering techniques such as LC (inductor-compacitor) networks require use of large and bulky inductors since the frequency range of interest is restricted to the common power line frequencies of 50 Hz and 60 Hz. These networks, like the RC networks, contain a component in their response which is dependent on the frequencies present in the incoming waveforms.
There remains a need therefore for apparatus for determining when two periodic waveforms are within a predetermined phase angle of being in phase which is not frequency dependent and which does not require large bulky components.